The invention pertains to a carriage for a machine tool.
In a machine tool such as a traveling column machining center or milling machine, a column supporting a rotary spindle is carried upon a cross-slide or saddle. Movement of the column along an axis is guided by means of ways mounted on the saddle. The saddle is guided along another axis, orthogonal to the first axis, by means of ways mounted upon a base or bed. Typically, the base is in contact with the floor of a work area, or like solid horizontal surface.
In a common configuration, the saddle is movably supported upon way bearing surfaces which lie in a common horizontal plane. The base has an effective depth comprising the distance between such plane and the floor supporting the base, and the saddle has an effective depth comprising the distance between the plane and the upper surface of the saddle, adjacent to the column. The distance between the floor and upper surface of the saddle, referred to as the "stacking height", is thus substantially the sum of the base depth and the saddle depth. Generally, the thrust resisting capability of a machine tool, that is, its ability to solidly take up thrust forces generated in the course of machining, increases as the base depth increases. On the other hand, the rigidity or stiffness of the saddle, and therefore its load carrying capability, is increased by increasing saddle depth.
In designing traveling column machines of the above type, and particularly large-size machines, the maximum acceptable level of stacking height will tend to be limited by certain practical considerations. For example, it could become awkward for an average size operator to perform certain tasks at a level above the stacking height, if the stacking height was higher than his shoulders. However, if a maximum limit is placed upon the stacking height, the depth of the base cannot be increased, to enhance thrust resisting capability, without correspondingly reducing the depth of the saddle. As saddle depth is decreased, reducing the stiffness thereof, downward deflection of the spindle becomes an increasingly serious problem, particularly when the column supporting the spindle is positioned midway between the ways supporting the saddle. In some designs, it is necessary to use three ways to support the saddle rather than just two, to prevent an unacceptable level of spindle deflection. Conversely, increasing saddle depth in a design requiring fixed stacking height correspondingly reduces base depth, and therefore thrust resisting capability.